Light offers many advantages when used as a medium for propagating information, the foremost of which are increased speed and bandwidth. In comparison with electrical signals, signals transmitted optically can be switched and modulated faster and can include an even greater number of separate channels multiplexed together. For these, as well as other reasons, light wave transmission along optical fibers is widespread.
Light can be propagated through planar waveguide structures as well as optical fibers. Planar waveguide structures having a wide variety of functionalities are currently available and many new such devices and components will likely result from future research and development. These planar structures are advantageous because they can be compactly incorporated together in or on a planar platform, i.e. substrate, to form planar packages analogous to integrated circuits (ICs). These structures in general are referred to as integrated optics. Integrated optical “chips” comprise a substrate on which or in which various integrated optical components or devices are formed. Planar waveguides analogous to conductor traces in semiconductor electronic ICs that are mounted in or on the substrate are employed to guide light to various optical, electro-optical, and optoelectronic devices or components on the chip.
One type of planar waveguide is referred to as a slab waveguide. Light can propagate within a planar slab of relatively high index material that is surrounded by a lower index medium. The light is confined within this slab by total internal reflection. A plurality of independent optical signals can be transmitted through the slab by constructing “ribs,” “ridges,” or “strips” that extend along one side of the slab. The optical signals propagating within the slab are localized to different regions within the slab that are defined by the ribs or strips. The ribs, ridges and strips thus provide independent pathways for light to travel in or on the planar substrate. Optical components on integrated optical chips can be interconnected in this fashion.
In many applications, it is desirable that the optical signal being transmitted through the waveguide be optically coupled into or out of the integrated optical chip. These signals may, for example, be coupled to optical fibers that are oriented out of the plane, i.e., above or below, the planar waveguide structure. The optical fibers may have dimensions that are significantly larger than the dimensions of the waveguide, that is, the optical fiber may have a diameter larger than the width or height of the planar waveguide. The disparity in size introduces loss in coupling of the optical signal between the optical fiber and the planar waveguide. What is needed, therefore, are designs and methods for increasing coupling efficiency in and out of planar waveguide structures.